In a wireless communication system, the transmitter and the receiver generally use multiple antennas to transmit and receive to obtain a higher rate. One principle of multi-antenna technology is to, based on the characteristics of the channel, form multi-layer transmission that matches some characteristics of the channel. The radiation direction of the signal is very targeted, can effectively improve the system performance, and obtain significant performance improvement without increasing the bandwidth and power. The performance improvement is a very promising technology that is widely used in the current system. The data transmission performance of a multi-antenna system mainly depends on the measurement and feedback of channel state information (CSI). Therefore, the measurement and feedback of CSI is the core content of multi-antenna technology. How to guarantee the accuracy, and the cost and robustness of the channel measurement and CSI feedback becomes an important issue.
The measurement and feedback of CSI is relatively simple to design in the early version of LTE (Long Term Evolution) system, but as precision requirements become higher and higher, while it is not desirable to have significant pilot overhead, feedback overhead, and quantization complexity. As a result, CSI measurement and feedback technologies have become more and more complex to pursue higher quantization efficiency. In addition, due to the need for adaptability to various scenarios and antenna configurations, a large number of new solutions have also been introduced. Here are some basic content related to CSI quantization feedback:
1. CSI (RI/PMI/CQI) Feedback Content
The channel state information includes: channel quality indication (abbreviated as CQI), precoding matrix indicator (abbreviated as PMI), and rank indicator (abbreviated as RI). CQI is an indicator of measuring the quality of the downlink channel. In the 36-213 protocol, the CQI is represented by an integer value of 0 to 15, which respectively represents different CQI levels. Different CQIs correspond to their respective modulation schemes and code rates (MCS). RI is used to describe the number of spatially independent channels, corresponding to the rank of the channel response matrix. In the open-loop spatial multiplexing and closed-loop spatial multiplexing modes, the UE (User Equipment, user equipment or terminal) needs to feed back RI information, and there is no need to feed back RI information in other modes. The rank of the channel matrix corresponds to the number of layers. The PMI feedback is the best precoding information, based on the index feedback, indicating the codeword of the agreed codebook that most closely matches the characteristics of the current channel. The standard supports configuration of a codebook used by each UE through a Codebook Subset Restriction function.
2. CSI Feedback
The feedback of the terminal CSI mainly exists in two ways: the base station may configure the terminal to measure and quantize the CSI, and perform periodical feedback on the quantized CSI information (including RI/PMI/CQI) through an uplink control channel (PUCCH). The base station can also suddenly trigger the terminal to report CSI information (including RI/PMI/CQI) non-periodically when needed, so as to overcome problems that the real-time performance of the periodic feedback is not high enough, and the CSI quantization accuracy is limited to the overhead of the control channel.
3. CSI Process
The 3rd Generation Partnership Project (3GPP) also introduces the concept of CSI process. The base station can configure a plurality of CSI processes for the terminal. Each CSI process is equivalent to a CSI measurement and feedback process, and each CSI process is independent to other CSI process and can configure CSI parameters separately. In the transmission mode 9, one process is supported; in the transmission mode 10, up to four processes can be supported. The configuration of the channel measurement section, the interference measurement section, and the feedback mode are defined in the configuration of each CSI process. The interference measurement section may be a single interference measurement configuration csi-IM-ConfigId, or may be an interference measurement list configuration csi-IM-ConfigIdList. The latter is mainly used when TDD (Time Division Duplexing) supports eIMTA. The configuration of the CSI process may also include some other configuration information, such as pilot power Pc information, bitmap indication information of a Codebook Subset Restriction, indication information selected by a 4Tx codebook version, and the like.
4. Channel Measurement Resources and Interference Measurement Resources
The channel measurement section generally specifies a set of non-zero power CSI measurement pilots (Non Zero Power CSI-RS) for channel measurement, and the interference measurement section generally specifies a set of IMR resource configuration or resource configuration list for interference measurement. The IMR resources generally can be a set of Zero Power CSI-RS or Zero Power CSI-RS list.
5. Feedback Categories
There are two categories of CSI measurement and feedback: Class A and Class B respectively.
Class A: The base station transmits the CSI-RS, which is generally a non-precoding pilot. The UE directly performs channel measurement and CSI quantization based on the CSI-RS pilot to obtain an RI/PMI/CQI. These contents are fed back on the PUCCH or PUSCH, and the feedback contents include, among others, the wideband beam direction.
Class B: The CSI-RS sent by the base station is generally a precoding pilot, and the UE may need to perform precoding pilot selection first, or pre-encode pilot resource set selection, or port group selection, and then the CSI quantization feedback is performed based on the selected subset, including subset selection information, and RI/PMI/CQI information corresponding to the selected CSI-RS measurement resource subset.
6. Introduction on Codebook Feedback
The basic principle of codebook-based CSI quantization feedback is briefly described as follows. Assuming that the limited feedback channel capacity is B bps/Hz, the number of available codewords is N=2B. The feature vector space of the channel matrix is quantized to form the codebook space ={F1,F2ΛFN}. The transmitter and the receiver both save or generate this codebook in real time  (for both of the transmitter and the receiver). To implement H for each channel, the receiver selects a codeword {circumflex over (F)} that most closely matches the channel implementation H from the codebook space  according to certain criteria and feeds back back the sequence number i (codeword number) of the codeword {circumflex over (F)} to the transmitter. Here, the codeword number is referred to as a precoding matrix indicator (abbreviated as PMI) in the codebook. The transmitter finds the corresponding precoded codeword {circumflex over (F)} according to this sequence number i to obtain the corresponding CSI, and {circumflex over (F)} represents the feature vector information of the channel. Here, the channel implementation H is generally obtained by channel measurement based on channel measurement pilots.
In general, the codebook space  can be further divided into multiple codebooks corresponding to Ranks, and each Rank corresponds to multiple codewords to quantize the precoding matrix formed by the channel feature vectors under the Rank. Since the number of Ranks and non-zero feature vectors of the channel is equal, generally, when the Rank is N, the codeword will have N columns. Therefore, the codebook space  can be divided into multiple subcodebooks according to the Rank, as shown in Table 1.
TABLE 1codebook divided into sub-codebooks by Rank layers ν (Rank)12. . .N  1  2. . .  Ncodewords vectorcodewords vector. . .codewords vectorset of 1 columnset of 2 columnsset of N columns
When Rank>1, the codewords that need to be stored are all in the form of a matrix, and the codebook in the LTE protocol is the feedback method used in this codebook quantization. The LTE downlink 4 transmits the antenna codebook, and actually the meaning of the precoding codebook and the channel state information quantization codebook in LTE is the same. In the following, for the sake of consistency, the vector can also be seen as a matrix with 1 dimension. There may be multiple sets of codebooks for selection to perform quantization with different accuracies and in different scenarios.
7. Introduction on Codebook Subset Restriction and Selection Configuration Signaling
Codebook subset restriction (abbreviated as CSR) is a kind of codebook configuration signaling, which can also be called Codebook Subset Selection. It refers to restricting a UE's codeword set to a subset of a large codebook set. In this way, codebook subsets suitable for the UE or codebook subsets that reduce interference to other UEs may be selected according to the channel characteristics of the UE, which can reduce the codebook selection complexity of the UE, reduce the overhead of the codebook feedback, and limit the beam direction of the UE. This method has obvious advantages for FD codebooks since FD-MIMO has a large number of antennas, codebooks with different numbers of antennas, codebooks with different antenna topologies. If one codebook is designed for each of the different numbers of antennas and for each of the different antenna topologies, the set of codebooks will be very huge. On the other hand, a large codebook set may be designed for the CSR, and then, based on the antenna number and topology of the base station as well as the UE's channel characteristics, one codebook set is given to the UE. In this way, the feedback overhead and the complexity for the UE searching codeword can be reduced.
8. Feedback Mode
The feedback mode refers to an instruction combination of CSI (CQI/PMI/RI) feedback, including subband feedback and wideband feedback or selecting M subband feedbacks, and including periodic feedback and non-periodic feedback. The non-periodic feedback is transmitted in the PUSCH, including the modes shown in Table 2:
TABLE 2PMI Feedback TypeNo PMISingle PMIMultiple PMIPUSCHWidebandMode 1-2CQI(wideband CQI)FeedbackUE SelectedMode 2-0Mode 2-2Type(subband CQI)HigherMode 3-0Mode 3-1Mode 3-2Layer-configured(subband CQI)
The periodic feedback mode refers to the mode of feedback in the PUCCH periodically, which includes the modes shown in Table 3:
TABLE 3PMI Feedback TypeNo PMISingle PMIPUCCH CQIWidebandMode 1-0Mode 1-1Feedback Type(wideband CQI)UE SelectedMode 2-0Mode 2-1(subband CQI)
9. RI/PMI Disabling
Channel rank indication disabling (abbreviated as RI disabling) refers to whether to report the rank of the MIMO channel when the UE reports a CSI in a multiple input multiple output system. In the scenario of using the uplink channel reciprocity to obtain the downlink channel, such as open loop spatial multiplexing, spatial subset, or TDD, the UE does not need to feed back RI, and thus RI disabling can be enabled, thereby saving feedback overhead.
Precoding matrix indicator disabling (PMI disabling) refers to whether to report the precoding index of the MIMO when the UE reports a CSI in a multiple input multiple output system. In the scenario of using the uplink channel reciprocity to obtain the downlink channel, such as open loop spatial multiplexing, spatial subset, or TDD, the UE does not need to feed back PMI, and thus PMI disabling can be enabled, thereby saving feedback overhead.
10. Introduction on Feedback Dimension
The quantized CSI dimension of the feedback has several situations. For example, the base station configures K sets of pilots in a CSI process, each set of pilots corresponds to a number Nk of CSI-RS resource sets, and the PMI in the CSI has a dimension L (the number of rows of the vector or matrix) may be equal to the total number of ports of a set of CSI-RS configurations: L=sum(Nk) for all k, or L=Nk, or L<Nk (signaling configured by the base station or determined by the UE); different feedback dimensions have different CSI quantization complexity and overhead.
11. Introduction on Measurement Limit
Measurement Restriction (MR) refers to that the UE can only perform channel measurement and joint operation in a time window or frequency domain size when performing channel measurement. These joint operations may be the average of the channel, LMMSE interpolation and so on. The time window refers to K consecutive subframes. The CSI-RS and other pilots in the K consecutive subframes have the same precoding. The frequency domain size refers to M consecutive physical resource blocks (PRBs) in the frequency domain. Here, the CSI-RSs in the PRBs have the same precoding matrix. K and M are positive integers. The MR can also be considered in the frequency domain. For example, the size of the frequency domain window of the joint operation is multiple resource block (RB) subbands and so on.
12. CSI-RS Pilot Configuration:
BFed CSI-RS may be implemented by multiple configuration methods, mainly including the following situations:
Configuring multiple sets of CSI-RS configurations. One beam can correspond to a set of CSI-RS configurations. This method is flexible, and can configure different pilot parameters (periods, number of ports, power parameters, etc.) for different configurations, but the configuration signaling overhead is large. Each CSI-RS configuration includes multiple ports. These ports generally have the same precoding, and the terminal selection beam is a selected CSI-RS configuration, as shown in FIG. 1(a).
Configuring a set of CSI-RS configurations, but including multiple resource sets. One beam can correspond to one resource set. This mode is less flexible, and cannot configure different pilot parameters (periods, number of ports, power parameters, etc.) for different configurations, but the configuration signaling overhead is small, the different resource sets correspond to different beams. The resource set can contain multiple ports, and these ports generally have the same precoding. The terminal selection beam is to select the resource set, as shown in FIG. 1(b).
Configuring a set of CSI-RS configurations as one resource set, but including a plurality of port groups. Different port groups have different corresponding beams, and a port group may include one or more ports. These ports generally have the same precoding. The terminal selecting the beam is to select the port group, as shown in FIG. 1(c).
The selection of the BFed CSI-RS generally requires feedback of a beam index, and the Beam Index is generally one or more; the latter has a larger overhead and less application scenarios.
The configuration of the non-precoding pilot may be a pilot configured with a set of total port Nt ports, including one resource set, and may also configure K pilot resource sets each having a number Nk of ports, and merge them into non-preconfigured pilots of Nt ports.
The pilot power parameter Pc refers to the ratio of the power transmitted by the CSI RS pilot to the power of the data transmitted in the same RB. The boosting value of the CSI-RS pilot with respect to the data can be known through Pc, so that the CQI on the data can be obtained by conversion when calculating the CQI.
Pilot multiplexing mode configuration: there may be multiple multiplexing modes for different pilot ports, For example, multiplexing by Code Division Multiplexing (CDM), with the orthogonal code length being 2, or multiplexing by code division multiple access CDM, with the orthogonal code length being 4.
The frequency domain position configuration sent by the pilot may be configured by multiple configurations, such as sending the RB position at full bandwidth, ½ bandwidth, even RB, base RB, or the RB where the data channel is located.
Bundling parameter of the pilot. Bundling means that the same precoded pilot (CSI-RS/DMRS) is bound with a certain frequency domain granularity or time domain window. In this frequency domain granularity, corresponding joint channel estimation or joint interference measurement estimation can be performed. The base station can configure the parameters such as bundling, bundling granularity, etc. for the pilot.
However, the related art has the following problems: a configuration method is to bind some of the above parameters with the configuration of the Process, or perform binding configuration with the CSI-RS configuration. The relationship between the above parameters and the CSI Process and CSI-RS configuration is determined during high-level configuration. One CSI Process/one set of CSI-RS configurations can only produce one relationship with the above parameters, which greatly limits the configuration flexibility, and makes a lot of flexible standard transparent technologies unable to be implemented.
This section provides background information related to the present disclosure which is not necessarily prior art.